There are many disease states and abnormal conditions that cause defects in the skeleton. For instance, osteoporosis and other metabolic bone conditions weaken the bone structure and predispose the bone to fracture. These same diseases also impair and prolong healing, which can lead to the formation of bone defects. If not treated, certain fractures and bone defects may progress and lead to the development of severe neurological or other medical complications.
Other examples of bone defects are those resulting from the excision of benign or malignant lesions of the skeleton. Removal of tumors often compromises the structural integrity of the bone structure and thus requires surgical stabilization and filling of the defects with biological materials such as bone grafts or cements.
Bone defects also result from bone grafting procedures, wherein the patient's own bone is transplanted to another region of the skeleton. Healing of the defect is often retarded and painful, necessitating further treatment including filling the defect with bone substitute materials to induce healing. If not repaired, the defect may worsen or fracture due to the compromise of structural integrity of the bone.
One approach to treating many bone defects comprises injecting, packing, or filling the defect with biocompatible bone cement. Such bone cements are generally formulations of non-resorbable biocompatible polymers such as PMMA (polymethylmethacrylate) or resorbable calcium phosphate or calcium sulphate cement. These cements allow the gradual replacement of the cement with living bone. Bone cements have been used successfully in the treatment of bone defects secondary to compression fractures of the distal radius, the calcaneous, the tibial plateau, and the vertebral body.
Historically, however, most applications of bone cements have been limited to open procedures in which the surgeon injects, packs, or tamps the biological material under direct visualization of the defect margins. Although direct visualization maximally allows the surgeon to identify adjacent structures that may be compromised by the inadvertent placement or injection of cement, less invasive means (apparatus and techniques) to assist the surgeon in safely and effectively placing biocompatible cements are generally desirable.
For example, one debilitating condition for which less invasive means to treat with injectable cement would be desirable is osteoporotic compression fracture of the spine. More than 700,000 osteoporotic compression fractures of the vertebrae occur each year in the United States—primarily in the elderly female population. Until recently, treatment of such fractures was limited to conservative, non-operative therapies such as bed rest, bracing, and medications.
A relatively new procedure known as “vertebroplasty” was developed in the mid 1980's to address the inadequacy of conservative treatment for vertebral body fracture. This procedure involves injecting radio-opaque bone cement directly into the fracture void through a minimally invasive cannula or needle under fluoroscopic control. The cement is pressurized by a syringe or similar plunger mechanism, thus causing the cement to fill the void and penetrate the interstices of broken trabecular bone. Once cured, the cement stabilizes the fracture and reduces pain—usually dramatically and immediately.
An alternative technique which has gained popularity in recent years is a modified vertebroplasty technique in which a “balloon tamp” is inserted into the vertebral body via a cannula approach to expand the fractured bone and create a void within the cancellous structure. The tamping effect is caused by the inflation of a balloon membrane that expands, thereby producing radial force. When subsequently deflated, the membrane leaves a void that is then filled with bone cement.
Regardless of which of these (or other) techniques is used when correcting defects within the vertebral body, it is generally desirable to inject cement substantially symmetrically or bilaterally to strengthen the entire vertebral body. In order to treat bilaterally, separate approaches to and access into the vertebral body have needed to be made from either side of the spine. Even for the simplest procedures, however, such vertebral approach and access requires skilled, delicate, time-consuming placement of the surgical instruments. Therefore, instrumentation and techniques that would facilitate surgical access to both sides of the vertebral body via a single approach is desirable.